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Chapter 13
Suggested Lecture Outline
I. INTRODUCTION
A. The spinal cord and spinal nerves mediate reactions to environmental changes.
B. The spinal cord has several functions.
1. It processes reflexes.
2. It is the site for integration of EPSPs and IPSPs that arise locally or are
triggered by nerve impulses from the periphery and brain.
3. It is a conduction pathway for sensory, to the brain, and motor
impulses to effectors.
II. SPINAL CORD ANATOMY
A. The spinal cord is protected by two connective tissue coverings, the meninges
and vertebra, and a cushion of cerebrospinal fluid.
1. The vertebral column provides a bony covering of the spinal cord
(Figure 13.1).
2. Meninges
a. The protective meninges are three coverings that run
continuously around the spinal cord and brain (Figures 13.1a,
14.2a, respectively).
1) The outermost layer is the dura mater
2) The middle layer is the arachnoid.
3) The innermost meninx is the pia mater,
3. The subarachnoid space carries cerebrospinal fluid, (CSF).
4. Denticulate ligaments are thickenings of the pia mater that suspend the
spinal cord in the middle of its dural sheath.
5. A spinal tap is done to withdraw CSF for diagnostic purposes (Clinical
Connection)
B. External Anatomy of the Spinal Cord
1. The spinal cord begins as a continuation of the medulla oblongata and
terminates at about the second lumbar vertebra in an adult (Figure
13.2).
2. It contains cervical and lumbar enlargements that serve as points of
origin for nerves to the extremities.
3. The tapered portion of the spinal cord is the conus medullaris, from
which arise the filum terminale and cauda equina.
4. Spinal nerves
a. The 31 pairs of spinal nerves are named and numbered
according to the region and level of the spinal cord from which
they emerge (Figure 13.2).
b. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves,
5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of
coccygeal nerves.
c. Spinal nerves are the paths of communication between the
spinal cord and most of the body.
d. Roots are the two points of attachment that connect each spinal
nerve to a segment of the spinal cord (Figure 13.3).
1) The posterior or dorsal (sensory) root contains sensory
nerve fibers and conducts nerve impulses from the
periphery into the spinal cord; the posterior root
ganglion contains the cell bodies of the sensory neurons
from the periphery.
2) The anterior or ventral (motor) root contains motor
neuron axons and conducts impulses from the spinal
cord to the periphery; the cell bodies of motor neurons
are located in the gray matter of the cord.
C. Internal Anatomy of the Spinal Cord
1. The anterior median fissure and the posterior median sulcus penetrate
the white matter of the spinal cord and divide it into right and left sides
(Figure 13.3).
2. The gray matter of the spinal cord is shaped like the letter H or a
butterfly and is surrounded by white matter.
a. The gray matter consists primarily of cell bodies of neurons
and neuroglia and unmyelinated axons and dendrites of
association and motor neurons.
b. The white matter consists of bundles of myelinated axons of
motor and sensory neurons
3. The gray commissure forms the cross bar of the H-shaped gray matter.
4. In the center of the gray commissure is the central canal, which runs
the length of the spinal cord and contains cerebrospinal fluid.
5. Anterior to the gray commissure is the anterior white commissure,
which connects the white matter of the right and left sides of the spinal
cord.
6. The gray matter is divided into horns, which contain cell bodies of
neurons.
7. The white matter is divided into columns.
a. Each column contains distinct bundles of nerve axons that have
a common origin or destination and carry similar information.
b. These bundles are called tracts.
1) Sensory (ascending) tracts conduct nerve impulses
toward the brain.
2) Motor (descending) tracts conduct impulses down the
cord.
8. The internal organization of the spinal cord allows reflexes to be
processed and to inform the brain of the results of those reflexes
(Figure 12.4)
9. Table 13.1 reviews the cross section of the spinal cord at different
segments.
III. SPINAL NERVES
A. Spinal nerves connect the CNS to sensory receptors, muscles, and glands and
are part of the peripheral nervous system.
1. The 31 pairs of spinal nerves are named and numbered according to
the region and level of the spinal cord from which they emerge (Figure
13.2).
2. Roots of the lower lumbar, sacral, and coccygeal nerves are not in line
with their corresponding vertebrae and thus form the cauda equina
(Figure 13.2).
3. Spinal nerves connect to the cord via an anterior and a posterior root
(Figure 13.3a). Since the posterior root contains sensory axons and the
anterior root contains motor axons, a spinal nerve is a mixed nerve, at
least at its origin.
B. Connective Tissue Covering of Spinal Nerves
1. Spinal nerve axons are grouped within connective tissue sheathes
(Figure 13.5).
a. A fiber is a single axon within an endoneurium.
b. A fascicle is a bundle of fibers within a perineurium.
c. A nerve is a bundle of fascicles within an epineurium.
2. Numerous blood vessels are within the coverings.
C. Distribution of Spinal Nerves
1. Shortly after passing through its intervertebral foramen, a spinal nerve
divides into several branches; these branches are known as rami
(Figure 13.6).
2. Branches of a spinal nerve include the dorsal ramus, ventral ramus,
meningeal branch, and rami communicantes.
3. The anterior rami of spinal nerves T2-T12 do not enter into the
formation of plexuses and are known as intercostal or thoracic nerves.
a. These nerves directly innervate structures they supply in the
intercostal spaces.
b. Their posterior rami supply the deep back muscles and skin of
the posterior aspect of the thorax.
4. The ventral rami of spinal nerves, except for T2-T12, form networks
of nerves called plexuses (Figure 13.2 and Exhibits 13.1-13.4).
a. Emerging from the plexuses are nerves bearing names that are
often descriptive of the general regions they supply or the
course they take.
b. The cervical plexus supplies the skin and muscles of the head,
neck, and upper part of the shoulders; connects with some
cranial nerves; and supplies the diaphragm (Figure 13.7,
Exhibit 13.1).
1) Damage to the spinal cord above the origin of the
phrenic nerves (C3-C5) causes respiratory arrest.
2) Breathing stops because the phrenic nerves no longer
send impulses to the diaphragm (Clinical Connection)
c. The brachial plexus constitutes the nerve supply for the upper
extremities and a number of neck and shoulder muscles
(Figures 13.8, Exhibit 13.2).
1) A number of nerve disorders may result from injury to
the brachial plexus (Figure 13.9)
2) Among these injuries are Erb-Duchene palsy or
waiter’s tip palsy, ulnar and radial injuries, wrist drop,
claw hand, and winged scapula (Clinical Connection).
d. The lumbar plexus supplies the anterolateral abdominal wall,
external genitals, and part of the lower extremities (Figure
13.10, Exhibit 13.3).
1) The largest nerve arising from the lumbar plexus is the
femoral nerve.
2) Injury to the femoral nerve is indicated by an inability
to extend the leg and by loss of sensation in the skin
over the anteromedial aspect of the thigh.(Clinical
Connection)
3) Obturator nerve injury is a common complication of
childbirth and results in paralysis of the adductor
muscles of the leg and loss of sensation over the medial
aspect of the thigh.(Clinical Connection)
e. The sacral plexus supplies the buttocks, perineum, and part of
the lower extremities (Figure 13.11, Exhibit 13.4).
1) The largest nerve arising from the sacral plexus (and
the largest nerve in the body) is the sciatic nerve.
2) Injury to the sciatic nerve (common peroneal portion)
and its branches results in sciatica, pain that extends
from the buttock down the back of the leg (Clinical
Connection)
3) Sciatic nerve injury can occur due to a herniated
(slipped) disc, dislocated hip, osteoarthritis of the
lumbosacral spine, pressure from the uterus during
pregnancy, or an improperly administered gluteal
injection.
D. Dermatomes
1. The skin over the entire body is supplies by spinal nerves that carry
somatic sensory nerves impulses into the spinal cord.
2. All spinal nerves except C1 innervate specific, constant segments of
the skin; the skin segments are called dermatomes (Figure 13.12).
3. Knowledge of dermatomes helps a physician to determine which
segment of the spinal cord or which spinal nerve is malfunctioning.
IV. SPINAL CORD PHYSIOLOGY
A. The spinal cord has two principal functions.
1. The white matter tracts are highways for nerve impulse conduction to
and from the brain.
2. The gray matter receives and integrates incoming and outgoing
information to perform reflexes.
B. Sensory and Motor Tracts
1. Figure 13.13 shows the principal sensory and motor tracts in the spinal
cord.
2. Sensory information from receptors travels up the spinal cord to the
brain along two main routes on each side of the cord: the
spinothalamic tracts and the posterior column tract.
3. Motor information travels from the brain down the spinal cord to
effectors (muscles and glands) along two types of descending tracts:
direct pathways and indirect pathways.
C. Reflexes and Reflex Arcs
1. The spinal cord serves as an integrating center for spinal reflexes. This
occurs in the gray matter.
2. A reflex is a fast, predictable, automatic response to changes in the
environment that helps to maintain homeostasis.
3. Reflexes may be spinal and cranial in location, and somatic, or
autonomic in function.
D. Reflex Arc
1. A reflex arc is the simplest type of pathway; pathways are specific
neuronal circuits and thus include at least one synapse.
2. The five functional components of a reflex arc are the receptor,
sensory neuron, motor neuron, integrating center neuron, and effector
(Figure 13.14).
3. Reflexes help to maintain homeostasis by permitting the body to make
exceedingly rapid adjustments to homeostatic imbalances.
4. Somatic spinal reflexes include the stretch reflex, tendon reflex, flexor
(withdrawal) reflex, and crossed extensor reflex; all exhibit reciprocal
innervation.
a. Stretch Reflex
1) The stretch reflex is ipsilateral and is important in
maintaining muscle tone and muscle coordination
during exercise (Figure 13.15).
2) A two-neuron or monosynaptic reflex arc contains one
sensory neuron and one motor neuron. A stretch reflex,
such as the patellar reflex, is an example.
3) It operates as a feedback mechanism to control muscle
length by causing muscle contraction.
b. Tendon Reflex
1) The tendon reflex is ipsilateral and prevents damage to
muscles and tendons as a result of stretching (Figure
13.16).
2) It operates as a feedback mechanism to control muscle
tension by causing muscle relaxation when muscle
force becomes too extreme.
c. Flexor and Crossed Extensor Reflexes
1) Flexor or Withdrawal Reflex
a) The flexor (withdrawal) reflex is ipsilateral and
is a protective withdrawal reflex that moves a
limb to avoid pain (Figure 13.17).
b) This reflex results in contraction of flexor
muscles to move a limb to avoid injury or pain.
c)
It works with the crossed extensor reflex to
maintain balance.
2) Crossed Extensor Reflex
a) This is a balance-maintaining reflex that causes
a synchronized extension of the joints of one
limb and flexion of the joints in the opposite
limb (Figure 13.18.
b) The crossed extensor reflex, which is
contralateral, helps to maintain balance during
the flexor reflex.
5. Reflexes are often used for diagnosing disorders of the nervous system
and locating injured tissue. (Clinical Connection)
a. If a reflex is absent, or abnormal, the damage may be
somewhere along a particular conduction pathway.
b. Among the clinically important reflexes are the Patellar reflex,
Achilles reflex, Babinski reflex, Abdominal reflex and Pupillar
reflex.
V. DISORDERS; HOMEOSTATIC IMBALANCES
A. The damage that results from Traumatic injuries depends on the degree of
spinal cord section, and or compressions and the segments involved. Complete
transection results in paralysis of a variety of forms. Lessor effects include
incomplete paralysis to loss of sensation and motor function. Spinal shock
often occurs immediately after injury with associated swelling. Administration
of corticosteroids is one treatment to reduce the swelling.
B. Shingles is an acute infection of the peripheral nerves by the herpes zoster
virus; the virus migrates down peripheral nerves, causing pain, skin
discoloration, and a characteristic line of skin blisters.
C. Poliomyelitis (infantile paralysis or polio) is a viral infection characterized by
fever, headache, stiff neck and back, deep pain and weakness, and loss of
certain somatic reflexes. Paralysis is produced when the virus destroys motor
neuron cell bodies.